This research is directed toward the development of an ion- microprobe analyzer which will be useful in two applications in biology. (1) It will permit the mapping of stable isotopes, particularly tracers, in biological materials to an accuracy of 100 angstrom units in some cases, more commonly 500 angstrom units. It will even permit simultaneous mapping of several independant tracers. (2) It will analyze the ion fragments produced in scanning the surface of a section, and this opens the possibility of developing a useful technique for chemical analysis with a resolution of a few hundred angstroms. We propose to study the fragment spectrum to see if it is a useful indicator of the chemical composition of the spot being bombarded. Both objectives require the development of a high-resolution microprobe (less than 1000 angstrom units in diameter ) using a field ion emission source and a scheme for efficient introduction of the secondary ions into a mass spectrometer of mass resolution greater than 5000.